This invention relates to fuel cell systems and, in particular, to a catalyst assembly for use in gas oxidizers used in such systems.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. Fuel cells operate by passing a reactant fuel gas through the anode, while passing oxidizing gas through the cathode. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell forming a fuel cell stack.
Molten carbonate fuel cells (“MCFCs”) operate by reacting oxygen in the oxidizing gas and free electrons at the cathode to form carbonate ions, which migrate across the molten carbonate electrolyte to the anode to react with hydrogen and produce water, carbon dioxide and electrical power. In MCFCs and other high temperature fuel cells, oxidizing gas provided to the cathode needs to be heated to the operating temperature of the fuel cell stack. Some MCFC systems include an anode exhaust gas oxidizer downstream from the fuel cell anode, which receives anode exhaust gas from an anode and oxidizing gas from an oxidizing gas supply and combusts unused fuel in the anode exhaust to produce heated oxidizing gas suitable for use in the fuel cell cathode. In particular, a typical anode exhaust gas oxidizer includes an oxidizing catalyst assembly for oxidizing or combusting hydrogen, carbon monoxide and unreacted hydrocarbons in the anode exhaust. In some cases, the anode exhaust oxidizer also includes a mixer where the anode exhaust gas and the oxidizing gas are first mixed before being exposed to the oxidizing catalyst.
The anode exhaust gas leaving the MCFC anode typically contains electrolyte molecules in a gas phase which are released during MCFC operation from the electrolyte layer of the fuel cell into the anode exhaust stream. When the hot anode exhaust gas is mixed with the oxidizing gas, which is at a lower temperature, the electrolyte molecules in the exhaust stream are transformed from gas phase into solid electrolyte particulates. These electrolyte particulates are deposited on the walls of the mixer and at the inlet face of the oxidizer catalyst assembly. The electrolyte particulate deposits create a partial obstruction of the flow path of the gas mixture into and through the oxidizer catalyst, resulting in an increased build-up of pressure across the catalyst assembly and thus increasing the difference between the pressure of the anode exhaust stream and the cathode inlet stream. In addition, the blockage by the electrolyte deposits changes the flow distribution through the catalyst assembly, resulting in a larger difference in temperature distribution from one end of the assembly to the other.
The performance and efficiency of the fuel cell stack is sensitive to the pressure changes in the fuel cell assembly. Particularly, the increasing pressure difference between the anode and the cathode streams due to the aforementioned accumulation of electrolyte particulate deposits on the oxidizer catalyst assembly affects the thermal profile and voltage variations of the fuel cell stack. Moreover, electrolyte particulate deposits may deactivate the oxidizer catalyst which affects its hydrocarbon oxidation efficiency.
Currently, electrolyte particulate deposits are removed from the oxidizer catalyst assembly in order to maintain the pressure difference between the anode outlet and the cathode inlet streams constant. Conventionally, electrolyte particulates have been removed from the oxidizer catalyst by washing the catalyst with a solvent suitable for removal of alkali carbonate compounds. This method of electrolyte particulate removal requires shut down of the fuel cell plant and disassembling of the oxidizer assembly to remove the oxidizer catalyst. In addition, U.S. patent application Ser. No. 11/022,914 discloses several methods of in-situ electrolyte particulate removal which do not require removal of the oxidizer catalyst from the oxidizer assembly. However, even the in-situ removal methods described in the '914 application usually require the fuel cell plant to be temporarily taken off-line, thus interrupting power generation and delivery.
It is therefore an object of the present invention to provide an oxidizer catalyst assembly which reduces electrolyte particulate build-up on the surface of the assembly.
It is a further object of the present invention to provide an oxidizer catalyst assembly which delays blocking of the gas flow through the oxidizer catalyst assembly by accumulated electrolyte particulates.
It is another object of the present invention to provide an oxidizer catalyst assembly which can be used together with the electrolyte removal practices described in the '914 application.